How do I join two separate objects so I can edit them as one mesh?
How to join two meshes with the Join command

It is very easy to join two meshes together using the Join (CTRLJ) command so that you can add a face between them. 

Here is how you do it: Select both meshes at the same time, then press CTRLJ:



Now both meshes are one object and in Edit mode, you can add a face between them.